RAM: there is no such thing as ENOUGH memory. What was sufficient when you bought it, is likely to be too little today.
Do you know how much RAM the PC has at the moment? When the machine boots up, it shows the size of RAM on the screen...
Find out what and how much RAM your PC's mainboard can handle. Look in the documentation of the PC or board. If there is no documentation available, pull all power plugs (machine assumed switched off), open case, and look for a type name on the mainboard (the thing where all the stuff is sticking out from).
Find out manufacturer (e.g. Asus), model/type and revision. On the WWW, all manufacturers offer support - look for the site in question (e.g.
http://www.asus.com ). You can find all documentation to your board there.
If necessary, print the details concerning memory/RAM for your type of board, and take it to a PC parts shop. Show them what it says, and see what they'll recommend.
Note: Since you upgrade, in most cases you might want to upgrade to the maximum, getting the most out of your effort. No use to do half measures now, and go back to do something serious in 6 months.